Powder metallurgy is a technique in which a raw material consisting of a metallic powder is compacted to have predetermined shape and size, and this is then sintered by heating to a temperature not causing melting, thereby strongly bonding the powder particles to each other, and this has advantages in that a near-net shape can be formed, it is suitable for mass production, special materials which cannot be made of the ingot material can be produced, or the like, and it can be applied to machine parts for automobiles and machine parts for various industries.
Sintered members generally have lower strength than ingot materials since gaps are formed between powder particles when a raw powder is compacted and remain as pores after sintering. For example, speed reduction starters are widely used as engine starting devices for automobiles. The speed reduction starter includes a speed reduction device having an internal gear and a planetary gear to reduce revolution speed of a motor, and includes a pinion gear that is connected to an output shaft of the speed reduction device and is engaged with a ring gear of the engine, whereby the engine is started. In the starter, sintered parts are applied to internal reduction gear parts such as an internal gear and a planetary gear; however, application of sintered parts to the pinion gear have been deferred since sintered parts do not have sufficient strength for the pinion gear, to which a load of up to six times may be applied compared to that of internal reduction gear parts.
This problem has been addressed by strengthening a matrix by adding a large amount of alloying elements to a sintered member, that is, it has been addressed by making higher grade steel than that of ingot materials. However, there is a limit to the strengthening of the matrix by alloying, and the cost of raw materials increases as the prices of alloy elements increase.
On the other hand, strengthening the matrix of the sintered member by reducing number of pores or disappearing pores has been investigated. A liquid phase sintering method is a technique in which an element which generates a liquid phase in sintering is added, whereby pores are filled with the liquid phase and disappear; however, it is difficult to ensure dimensional accuracy, whereby machining is required after sintering, and the advantage of powder metallurgy, in which a near-net shaped body can be formed, is reduced. A powder forging method in which a raw material powder is heated and forged in a heated die set or a sinter forging method in which a raw material formed by compacting and sintering through powder metallurgy is hot forged require heating apparatuses for heating the raw material powder and the die set or sintered compact, whereby the producing cost may increase.
In such circumstances, sintering and cold forging methods, in which a raw material formed by compacting and sintering through powder metallurgy is cold forged, thereby obtaining high density, has been researched (Patent Publications 1 and 2). A member produced by the sintering and cold forging method is often subjected to carburizing and quenching to obtain hardness and strength as the member as well as mechanical parts made from ingot materials. That is, if hardness of a mechanical part is uniformly increased, toughness is decreased. Therefore, carburizing and quenching are widely used as methods for improving toughness by maintaining the hardness of the inner portion low to a certain extent, as well as improving wear resistance and fatigue strength with respect to repeated surface pressure by hardening only the surface that slides with a counter member and is subjected repeatedly to surface pressure.
In carburizing and quenching, a heat treatment object is exposed to a carburizing gas atmosphere having a Cp (carbon potential, which is carbon concentration [mass %] in the atmosphere gas) which is higher than the carbon content of the matrix at a temperature which is approximately 100° C. higher than the austenite transformation temperature, whereby carbon is diffused from the gas atmosphere into the heat treatment object and required carburized layer is formed, then the heat treatment object is dipped into an oil, etc., and is rapidly cooled and quenched, thereby forming a hard matrix such as martensite and bainite. Then, the heat treatment object is subjected to tempering at a temperature of about 200° C. for a suitable time to improve toughness. In such a generic hardening method, the thickness of the carburized and hardened layer having a Vickers hardness value of 550 or more can be adjusted by increasing the quenching time. In this case, the distribution of hardness is such that the hardness of the outermost layer is highest and the hardness gradually decreases toward the inner portion.
Patent Publication 1 is Japanese Unexamined Patent Application Publication No. 2003-253372.
Patent Publication 2 is Japanese Unexamined Patent Application Publication No. 2001-513143.